Portosystemic shunts divert blood around the liver instead of through it, resulting in decreased liver functionality and increased levels of toxins. Hepatoportal microvascular dysplasia (HMD), otherwise known as microvascular dysplasia (MVD), is a type of liver shunt that occurs at a microscopic level and carries mild clinical signs, if any are present at all. These inherited liver abnormalities are most prevalent in small breed dogs.

Most dogs with hepatoportal microvascular dysplasia are asymptomatic, and any clinical signs that may be present are similar to those for portosystemic shunts. These include:

Symptoms are most prevalent following meals and may be exhibited early or develop later in life.

There are a number of liver abnormalities that may arise due to genetic disorders, but the two most common are:

Portosystemic shunts and microvascular dysplasia are both genetic disorders that are most prevalent in small or toy breed dogs, such as Cairn Terriers and Yorkshire Terriers. There is no known cause for this genetic predisposition.

When you visit the veterinarian, provide a list of your dog’s symptoms and their duration. The veterinarian may suspect a portosystemic shunt based on your dog’s clinical signs and history, and the diagnosis, in most cases, can be confirmed through in-house or external laboratory testing based on the veterinarian's office setup. A complete blood count may reveal signs that point towards PSS, such as higher liver enzymes, small red cells, and low blood urea nitrogen. Other tests include a biochemical analysis, urinalysis, and measurement of bile acids both before and after fasting.

Because dogs with hepatoportal microvascular dysplasia are often asymptomatic, the defect is usually diagnosed when tests are being done for other conditions. The results from laboratory testing are similar to those for PSS.

Further diagnostic tests can be performed to differentiate between PSS and MVD. An MRI or CT may reveal a portosystemic shunt. If none are detected, a liver biopsy may be necessary in order to confirm MVD.

Treatment is not always necessary depending on the impact and seriousness of the disease. Dogs with MVD may live full lives without ever requiring treatment, so long as no symptoms manifest. PSS, however, may progress to the point where it endangers the dog’s life if left untreated.

You may be able to manage the symptoms of PSS through nutritional supplements and changes in diet. By switching to a diet with higher quality protein, you can reduce the amount of toxins that are produced in the body. Antibiotics may be prescribed in order to decrease the number of bacteria in the large intestine though these do carry side effects and may not be ideal. Probiotics, such as the active cultures found in yogurt, can be beneficial to your dog’s digestive system.

Surgical shunt ligation is the treatment of choice for most cases of PSS, as medical management only addresses the symptoms and does not prevent the disease from progressing. With surgery, the veterinarian will be able to close the shunt either partially or fully, depending on the dimensions of the shunt and on your dog’s condition.

With both medical and surgical treatment, complications may arise. You will need to monitor carefully your dog following the surgery and return to the veterinarian immediately if you notice seizures, diarrhea, or other abnormal signs. If you chose to opt out of surgery, make sure to follow the veterinarian’s instructions regarding your dog’s diet, and watch for further symptoms of PSS. Medical management does not treat the shunt itself, and your dog may still be at risk for life-threatening health issues.

The prognosis for dogs with MVD is typically good though the success rate of surgical or medical treatment of PSS varies according to your dog’s age and condition. The majority of dogs who undergo surgical shunt ligation do survive long term.